A standard multiple-jaw lathe chuck has a base that is normally rotated about and centered on the lathe axis and that carries a plurality of angularly spaced supports to each of which is secured a respective jaw. Means is provided to move the supports radially inward to grip a workpiece and outward to release it. The jaws are removable from the supports so that they can be changes if they get damaged or a different workpiece or machining operation demands differently shaped jaws. Normally the jaws are secured in place by simple bolts.
Thus changing jaws can be a fairly difficult job, especially as the jaws can get frozen in place after being subjected to considerable stresses and heat during machining. What is more the mounting formations can deform so that the new jaws do not assume the proper positions relative to each other, which can result in eccentric mounting of the workpiece and offcenter machining.